Tokyo hosts 2 festivals in this guide: Summer Sonic, Ultra Japan.
Tokyo has 2 festivals in this guide. They run in August and September, and festival-day temperatures range from 28°C (82°F) to 32°C (90°F), with nights down to 21°C (70°F). That range is the reason there is no single "Tokyo festival outfit". The answer changes by month and by site.
Every one of them run in real heat (28°C or more), where loose natural fabrics beat coverage and a brimmed hat does more than any garment.
The ground matters as much as the sky. You will find arena grounds and city streets, which puts broken-in trainers or a low boot at the top of the list for most of these events. None of them have camping on site, so you are travelling in and out each day and the outfit has to survive the journey at both ends.
Stylistically the crowd here leans kawaii, j-pop, streetwear and urban. Summer Sonic is the largest of them at around 150,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.
